In the normal operation of an automotive vehicle, a 12-volt battery is used to start the engine and to operate accessories at times when the motor is not running. An alternator is used to recharge the battery after each start and to maintain it fully charged. Once the engine is started the alternator is driven by the rotation of the engine crankshaft.
Conventional automotive batteries have six cells connected in series with thin cell plates and are well suited to developing large starting currents of 240 amps. It has long been the practice, in order to insure that the battery is adequately charged following a start discharge, to maintain a recharging current flow to the battery at a level that varies as a function of ambient temperature. This results in trickle charge currents. These trickle charge currents cause prior known battery state of charge monitors to indicate erroneously greater than a 100% charge. This results in erroneous state of charge readings.
It is known to monitor the state of charge of a battery to determine, for example, whether a battery is fully charged, depleted of charge, or partially charged. One such technique calls for measuring the specific gravity of the electrolyte of each cell of the battery using a hydrometer. The determined specific gravity can be compared to the specific gravity of the battery in a fully charged state to obtain a relative state of charge.
Another technique uses a current counter which continuously registers the net current from automotive battery and is disclosed in the ELV Journal No. 45, dated May/June 1986. That technique measures the voltage drop across a precision shunt resistor in series with the monitored battery. The voltage is amplified and drives a voltage to frequency converter, the output of which is fed into an up/down counter. The counter counts up or down while the battery is discharged or being charged respectively, and may drive an integrated display. The display is initialized when the counter is first connected to a fully charged battery and displays the net current flow from the battery. For continuous monitoring, the display must be continuously energized by a power source so that the counter will not lose the net count through loss of power. U.S. Pat. No. 4,968,941, which issued to the assignee of the present invention, teaches a battery state of charge monitoring circuit ("BSOC circuit") including a shunt resistor in series with the battery and a circuit for integrating the voltage drop across the shunt having a very large capacitance in the feedback path. The large capacitance device, such as a Super Capacitor having a capacitance between 0.01 and 1 farad or more, integrates the current flow into and out of the battery over time, respectively, and thus maintains a charge at a level corresponding to the battery state of charge. The charge is maintained for long periods of time whether or not the element is connected to a power supply because of the long RC time-constant of the integrator amplifier circuit and a very low self-discharge rate of the capacitive device. The BSOC circuit may be connected to a display and initialized with a fully charged battery so that the voltage stored in the large capacitance device represents the state of charge of the battery. Alternatively, the BSOC circuit may be connected to a discharged battery such that the capacitive device is charged up with the battery.
The present invention concerns improvements to the basic BSOC circuit disclosed in U.S. Pat. No. 4,968,941, the disclosure of which is hereby incorporated by reference in its entirety.